1. Field of the Invention
This invention relates to a radiation dose reading apparatus for reading radiation dose from a fluorescent glass element, and particularly to an apparatus for estimating the dose of radiation, the quality (energy) of radiation and the incidance direction of radiation rays.
2. Description of the Prior Art
Generally, a fluorescent glass element made of phosphate glass containing silver ions is used for reading radiation dose. After the fluorescent glass element has been exposed to ionizing radiation rays to be activated and be excited by ultraviolet rays having a wavelength of 300 to 400 nm, fluorescence is emitted from a predetermined face of the fluorescent glass element. By using the fact that the intensity of the fluorescence is proportional to the radiation exposure dose, the radiation exposure dose is measured from the intensity of the fluorescence.
Conventionally, there have been developed fluorescent glass dosemeters employing this principle. A conventional dosemeter has a structure as shown in FIG. 19 such that workers at radiation facilities measure the radiation exposure dose with the dosemeter by weaving it (see Japanese Patent Application No. Sho 62-334649). This fluorescent glass dosemeter is of portable type and is constructed in such a way that, after a fluorescent glass element 2 has been inserted in a glass element holder 1, the holder 1 is housed in a lower case 3 at a side thereof as shown by an arrow (A), and thereafter the lower case 3 is put in an upper case 4 as shown by an arrow (B). Reference numeral 5 shows a filter made of tin and attached to the inner face of the lower case 3 in order to compensate for the dependency of energy on radiation dose. Similarly, a filter made of tin is attached to the inner face of the ceiling of the upper case 4.
When the workers at the radiation facilities measure the radiation exposure dose after wearing the fluorescent glass dosemeter, an the radiation dose is read, the lower case 3 is pulled out from the upper case 4, and then the glass element holder 1 is removed therefrom. Thereafter, as shown in FIG. 20, ultraviolet rays 6 of a specific wave length, selected by an optical filter (not shown) disposed in the path of the ultraviolet rays are emitted thereto from a light source of exciting ultraviolet rays. They are substantially perpendicularly incident on a face of the fluorescent glass element 2. Then, fluorescence 7 is generated in the fluorescent glass element 2 made of silver activated phosphate glass by the excitation of the ultraviolet rays 6, and is taken out in a direction perpendicular to that of the ultraviolet rays 6. The fluorescence 7 within a specific wavelength range is selectively picked up through another optical filter (not shown) and is photoelectrically converted by a photomultiplier tube or the like. The intensity of the fluorescence, that is, the radiation exposure dose can be read from the output of the photoelectronic converter.
When the fluorescent glass dosemeter is used to measure the radiation exposure dose of individual persons, it is normally sufficient to measure only the dose. However, when an emergency arises in the facilities, or when a leakage of radioactive substance occurs because of insufficient shielding, or when the operator works for a long time and is exposed to a large radiation dose, not only the dose but also the quality of radiation and the incidence direction of the radiation must be detected in order to estimate and analyse the individual exposure of workers.
As described above, the filter 5 having a slit formed therein is attached to the inner face of the case so as to face the radiation exposed (irradiated) face of the fluorescent glass element 2. This filter 5 is effectively utilized to estimate the quality of radiation. For example, two diaphragms are employed for the estimation. One diaphragm 8 is shown in FIG. 21 and is used to measure the substantially whole fluorescence of the fluorescent glass element 2 when it is normally used for reading a dose. The other diaphragm 9 is shown in FIG. 22 and measures only the fluorescence produced from the part of the fluorescent glass element 2 to which rays and X-rays are irradiated through the filter 5. These diaphragms 8 and 9 are alternately placed on the fluorescent glass element 2, and the amounts of fluorescence 7 concerning workers at (A) and the fluorescence 7 concerning workers at (B) are read which pass the diaphragms 8 and 9, respectively. The quality of radiation can be estimated by obtaining the ratio of the fluorescence 7 workers at (A) to the fluorescence workers at (B).
FIG. 23 shows relative energy response curves (A), (B), so-called energy dependence of response, when the fluorescence of fluorescent glass element 2, to which the same dose of T-rays rays and X-rays is irradiated, is detected using the diaphragms 8 and 9 respectively. As shown in FIG. 24, the relative response ratios can be obtained by calculation (A/B). Accordingly, the qualities of irradiated .gamma.-rays and X-rays can be estimated from the relative response ratios.